Generally, sheet articles having a porous structure may be classified into the following five groups according to pore size:
Pore Size (.mu.m) Materials Uses 0.01-0.1 Gas-Barrier Film Gas Permeability 0.1-1.0 Film Filter Water-Vapor Permeability 1.0-10.0 Meltblown, Flash- Biobarrier spinning Web 10.0-100 Conventional Nonwoven Porous Articles Fabric 100-1000 Perforated Nonwoven Perforated Articles Fabric
Among these porous sheets, those having a pore size of about 10 .mu.m or less are generally referred to as "microporous materials." Such sheets are produced according to special techniques for forming films, such as an extracting method, a phase separation method or a method which comprises adding an inorganic powder in a high concentration and stretching the film in a biaxial direction. Microporous sheets are widely applied to special filters, air permeable water-proof sports wear and the like. Microporous sheets are also used in composites in conjunction with nonwoven fabric and/or woven fabric.
For example, Japanese Patent KOKAI (Laid-Open) No. 14023/89 discloses a method of producing a porous sheet. There, a film of a crystalline polyolefine resin, a rubber-like polymer and an inorganic filler are first drawn and then hot-crimped to orient the film into a mesh-like sheet. The mesh-like sheet is then fixed and thermally contracted. The lack of uniformity of the mesh-like sheet causes deficient performance.
Further, porous sheets manufactured according to the conventional methods are generally hard and fragile. Consequently, porous sheets are not usually satisfactory as a high performance material for application in sanitary articles, medical articles, etc.
These and other drawbacks of the prior art are sought to be overcome by the porous composite sheet of the preferred embodiments.